Arguably the cell is the most fundamental biological unit. The cell is also the unit of ontological primacy in molecular biology.

The complexity of development is really about the complexity of the cell. Cells are much more complicated than embryos – what's going on in a cell is much more complicated than what's going on between cells in an embryo. And morphogenesis, which is about changes in form, is largely about cell mechanics, and the types of forces that cells generate are very few: tensions, occasionally extensions, and changes of neighbors.

Although morphogenesis appears to be quite deterministic on a macroscopic scale, on a microscopic scale cellular activities during the formation of the limb appear to be nearly random. Order emerges only as an average outcome, a biasing of many individual motions, each of which has a large stochastic component. Therefore, we can only speak of the probability of a certain developmental process occurring in a given setting.